To perform various treatments, such as film forming, etching and the like, on a semiconductor wafer which serves as a substrate, a substrate processing apparatus generally includes a plurality of processing modules (hereinafter, referred to as “PMs”), each of which can accommodate the wafer to conduct a predetermined process thereon; a wafer cassette mounting table mounting thereon a wafer cassette serving as an airtight container storing therein predetermined sheets of wafers; and transfer arms arranged as a substrate transfer mechanism between the PMs and the wafer cassette mounting table. The transfer arm transfers a wafer among the PMs, load lock module, and the wafer cassette mounting table.
In a substrate processing apparatus performing a plurality of processes on a wafer in which the wafer is transferred between various processing units, such as PMs, load lock module, or the like, it becomes difficult to precisely perform a process on the wafer if a relative misalignment occurs while the wafer is being transferred. Accordingly, the relative misalignment of the wafer needs to be prevented or corrected when transferring the wafer in the substrate processing apparatus that performs a plurality of processes on the wafer.
To prevent such a relative misalignment of the wafer, the substrate processing apparatus generally is provided with a pre-alignment unit conducting a positional alignment of the wafer before loading the wafer into the processing units. As a target to be processed, the wafer is position-aligned by the pre-alignment unit before being transferred to the processing units by the transfer arm. The transferred wafer is subjected to a predetermined process by each processing unit.
In order to achieve a precise position alignment of the wafer in a series of treatment processes, however, the wafer previously subjected to a first treatment in a first PM needs to be subjected to a position alignment again before being loaded into a next PM for the subsequent process. To this end, there has been developed a technology for misalignment correction in which a wafer sensor is installed at a predetermined position in a wafer transfer path to detect misalignment of the wafer passing by the sensor. The detected wafer is subjected to a misalignment correction process before being loaded into the next PM for the subsequent process.
Since the wafer is subjected to the misalignment correction process before being loaded into the next PM for the subsequent process, the technology does not require a feedback control for the purpose of misalignment correction when the wafer is loaded into the PM for the next process. However, the wafer needs to be returned to the position where the wafer sensor is installed in the transfer path whenever the wafer is loaded into a PM for subsequent process. This makes the operation cumbersome. For this reason, a wafer transfer method has been suggested in which the misalignment of the wafer is detected and corrected when the wafer unloaded from a first PM performing a certain treatment is loaded into a next PM performing another treatment.
For example, Japanese Patent Laid-open Application No. 2004-282002 discloses such a misalignment correction method, wherein the center of the wafer is monitored when a wafer unloaded from a processing chamber (chamber) of a PM is loaded into a chamber of a next PM, and the wafer is subjected to a misalignment correction process when being mounted on a substrate mounting table provided in the chamber of the next PM.
However, in the above-described prior art, in which the misalignment of a wafer is detected and then corrected by feedback control when the wafer is loaded into the processing chamber of the next PM, the misalignment correction is carried out immediately before reaching a destination position. Therefore it is difficult to secure a sufficient correction control time. Further in the prior art, further, the wafer once transferred to the destination position or its proximity needs to be accurately adjusted before being moved to a targeted position after correction, and this makes the operation cumbersome. Further, when the wafer is loaded into the chamber of the next PM, an arm of a transfer mechanism is generally in a fully extended state. Accordingly, if the misalignment correction is performed in such a state, it becomes difficult to achieve an accurate position alignment, thus making it difficult to accurately fulfill the position correction.